Devices for protecting electrical installations against surges are in widespread use, and are commonly referred to as “lightning arrestors”. Their essential purpose is to carry lightning currents to ground, and possibly also to limit the peak additional voltages induced by such currents to a level that can be withstood by the equipment and apparatuses to which they are connected.
It is already known to use a spark gap lightning arrestor to protect an installation against surges. The spark gap is then connected between the phases for protection and ground so that in the event of a surge, the lightning current can be carried to ground.
A spark gap is a well-known device comprising two electrodes placed facing each other and spaced apart in a dielectric medium. One of the electrodes is electrically connected to the phase for protection, while the other electrode is electrically connected to ground. In the event of a surge, as generated by the arrival of lightning current, reaching a so-called “trigger” threshold value, an electric arc is struck between the electrodes of the spark gap, thus creating a short-circuit between the phase and ground. The lightning current then flows from the phase to ground and the electrical installation is preserved.
The electric arc does not extinguish spontaneously and therefore continues to carry a short-circuit current, referred to as the “follow current”. This follow current should preferably be interrupted without opening general interrupter devices of the installation, such as circuit breakers, in order to avoid disconnecting the installation.
Spark gap lightning arrestors often have only one spark gap, associated with a pre-trigger system (a trigger electrode), and a control device that is sensitive to voltage and that is electrically connected to the pre-trigger system so as to activate it.
Although such single spark gap devices are advantageous because of their design which is particularly simple, they nevertheless present certain drawbacks, associated in particular with their limited capacity for carrying lightning current and also their limited capacity for breaking follow current.
To mitigate those drawbacks, it is known to connect two or more spark gaps in parallel so as to distribute the lightning current and the follow current better between the parallel-connected spark gaps, thus enabling the overall capacity of the lightning arrestor for carrying lightning current and for breaking follow current to be improved.
Such lightning arrestors with parallel-connected spark gaps generally rely on the following concept.
Two spark gaps are connected in parallel and a respective inductor coil is connected in series with each of them. Thus, when an arc is struck in one of the two spark gaps, the current flowing through the inductor coil connected in series therewith generates a voltage across its own terminals, which voltage is applied across the terminals of the second spark gap, thereby causing it to strike an arc.
Such protector devices, although they provide better protection than devices with a single spark gap, nevertheless suffer from several drawbacks.
Firstly, when the current flowing through the first spark gap is below a predetermined value, the voltage generated across the terminals of the inductor coil is not sufficient to strike an arc in the second spark gap. Under such circumstances, current is not distributed between the two spark gaps, and as a result the capacity of the device for carrying lightning current corresponds substantially to the capacity for carrying lightning current provided by the first spark gap on its own.
Conversely, when the magnitude of the current flowing through the first spark gap is high, the voltage across the terminals of the inductor coil becomes large and is added to the voltage across the terminals of the spark gap, thereby degrading the quality of the peak-limiting performed by the spark gap and thus degrading the level of protection it provides.
It can thus be seen that it would be advantageous to provide a surge protector device which, while being simple and inexpensive in design, nevertheless presents improved capacity for carrying lightning current and for breaking follow current.